The invention relates to a fuel injector.
When using a fuel injector, precisely metered quantities of fuel are injected into in a combustion chamber of an internal combustion engine. With regard to future common rail injection systems, the fuel is intended to be injected at a pressure of up to 2000 bar, for which reason efforts are being made to design fuel injectors capable of handling particularly high pressure.
A conventional fuel injector will be described in detail in the following with reference to FIG. 1 which shows a cross-section through the fuel injector.
The fuel injector comprises an actuator housing GAxe2x80x2 and an actuator unit Axe2x80x2 arranged therein which has an operative connection by way of a lever Hxe2x80x2 and a valve piston Vxe2x80x2 with a control valve Sxe2x80x2. The control valve Sxe2x80x2, which is arranged in a valve chamber VKxe2x80x2, separates a control chamber SKxe2x80x2 from a return line Rxe2x80x2. The control chamber SKxe2x80x2 is arranged beneath the valve chamber VKxe2x80x2 and connected by way of an outlet restrictor ADxe2x80x2 to the valve chamber VKxe2x80x2. The valve chamber VKxe2x80x2 is arranged in a control module STxe2x80x2.
The control chamber SKxe2x80x2 lies adjacent to an upper end of a control piston Kxe2x80x2. The control piston Kxe2x80x2 is arranged so as to be movable inside a drilled hole in a piston module KMxe2x80x2 and lies adjacent to side surfaces of the piston module KMxe2x80x2 which are formed by the drilled hole. The drilled hole thus serves as a guide for the control piston Kxe2x80x2.
The control piston Kxe2x80x2 is connected to a coupling rod KSxe2x80x2 which is arranged in a spring pocket Fxe2x80x2. The spring pocket Fxe2x80x2 is arranged in the piston module KMxe2x80x2 and is connected to the return line Rxe2x80x2 such that a low pressure exists in the spring pocket Fxe2x80x2. The coupling rod KSxe2x80x2 has a spring plate Txe2x80x2. A spring FExe2x80x2 is tensioned between the spring plate Txe2x80x2 and the control piston Kxe2x80x2.
The coupling rod KSxe2x80x2 is in contact with a nozzle needle Dxe2x80x2 which is arranged in a drilled hole in a nozzle body DKxe2x80x2 arranged beneath the piston module KMxe2x80x2. The drilled hole in the nozzle body DKxe2x80x2 has a high pressure chamber HKxe2x80x2 into which a high pressure inlet Zxe2x80x2 opens out which extends from the control module STxe2x80x2 as far as the high pressure chamber HKxe2x80x2. An inlet restrictor ZDxe2x80x2 is arranged between the high pressure inlet Zxe2x80x2 and the control chamber SKxe2x80x2.
When the actuator unit Axe2x80x2 is actuated, then the control valve Sxe2x80x2 is opened so that fuel drains from the valve chamber VKxe2x80x2 by way of the return line Rxe2x80x2. As a result, fuel flows from the control chamber SKxe2x80x2 by way of the outlet restrictor ADxe2x80x2 into the valve chamber VKxe2x80x2 and it actually flows more quickly than fuel flows from the high pressure inlet Zxe2x80x2 by way of the inlet restrictor ZDxe2x80x2 into the control chamber SKxe2x80x2. As a consequence of this, the pressure in the control chamber SKxe2x80x2 falls such that the force acting from above on the nozzle needle Dxe2x80x2 is reduced and the nozzle needle Dxe2x80x2 lifts from its valve seat. As a result, fuel issues from the fuel injector.
When the actuator unit Axe2x80x2 is deactivated, then the control valve Sxe2x80x2 closes so that a pressure is built up once again in the control chamber SKxe2x80x2 by way of the inlet restrictor ZDxe2x80x2. As a result of the spring FExe2x80x2, as a result of the low pressure in the spring pocket Fxe2x80x2 and as a result of the hydraulic force resulting on the basis of the greater cross-sectional area of the control piston Kxe2x80x2 when compared with the cross-sectional area of the nozzle needle Dxe2x80x2 in the area of the guide in the nozzle body DKxe2x80x2 just a small rise in pressure in the control chamber SKxe2x80x2 is sufficient in order to press the nozzle needle Dxe2x80x2 downwards against its valve seat such that the fuel injector closes quickly.
A disadvantage associated with the conventional fuel injector is the tapering and thin wall of the nozzle body in the area where the high pressure inlet opens out into the high pressure chamber. The resistance to high pressure of the fuel injector is consequently not very high.
A further disadvantage consists in the fact that a continuous leakage occurs between the high pressure chamber and the spring pocket in which a low pressure prevails, and between the spring pocket and the control chamber, which leads to a loss in the efficiency of the fuel injector. The greater the pressure difference between the high pressure chamber or the control chamber and the spring pocket, the more pronounced is the continuous leakage.
The object of the invention is to set down a fuel injector which is suitable for higher pressures when compared with the prior art.
This object can be achieved by a fuel injector having the following features: The fuel injector comprises a control module with a piston guide extending downwards, in which a control piston is arranged. The fuel injector further comprises a nozzle body with a top surface on which the control module is mounted and which has a drilled hole in whose lower section is arranged a nozzle needle which has an operative connection with the control piston and in whose upper section is arranged the piston guide of the control module. A high pressure inlet which opens out into the drilled hole at the top surface is arranged in the control module. The drilled hole is designed such that fuel which escapes from the fuel injector when the nozzle needle lifts from its valve seat is replaced, whereby fuel from the high pressure inlet flows through the drilled hole in the direction of the valve seat. High pressure is thus applied to the entire drilled hole.
Since the high pressure inlet opens out into the drilled hole of the nozzle body at the top surface of the nozzle body and thus does not open out sideways into a drilled hole, no tapering thin wall which would be at risk of failure under high pressure conditions is present between the drilled hole and the high pressure inlet. The fuel injector therefore exhibits a high resistance to pressure and is thus suitable for high pressures.
Since the high pressure inlet is arranged only in the control module and not in the nozzle body where the construction space particularly in the lower section is greatly restricted, the problem of walls which are too thin for high pressures around the high pressure inlet does not generally arise.
A valve chamber is provided, for example, which is separated from a return line by means of a control valve. In addition, the fuel injector can comprise a control chamber which lies adjacent to the upper end of the control piston. High pressure is applied to the control chamber by way of an inlet restrictor, whereby the inlet restrictor is connected hydraulically to the high pressure inlet. The inlet restrictor is thus connected at least indirectly to the high pressure inlet. The valve chamber and the control chamber are connected to one another by way of an outlet restrictor.
In order to guarantee rapid closure of the fuel injector, as a result of the absence of a difference in cross-sectional area between control piston and nozzle needle in the area of the guide in the control module and thus of the absence of the hydraulic force component in the direction of closure of the nozzle needle it is advantageous to provide a bypass restrictor, by way of which high pressure is applied to the valve chamber, whereby the bypass restrictor is connected hydraulically to the high pressure inlet. The bypass restrictor is thus connected at least indirectly to the high pressure inlet in hydraulic terms. When the control valve lifts from its valve seat, then fuel drains off from the valve chamber into the return line. Fuel drains off from the control chamber through the outlet restrictor more quickly than can flow into the control chamber through the inlet restrictor, which causes the pressure in the control chamber to fall, as a result of which the nozzle needle lifts from its valve seat and fuel issues from the fuel injector. At the same time, fuel flows into the valve chamber by way of the bypass restrictor. When the control valve is closed, then the pressure builds up in the control chamber as a result of fuel flowing through the inlet restrictor. The pressure buildup and thus the lowering of the nozzle needle onto its valve seatxe2x80x94in other words the closure of the fuel injectorxe2x80x94is accelerated by means of the bypass restrictor because fuel flows into the valve chamber by way of the bypass restrictor and thence by way of the outlet restrictor into the control chamber.
In order to ensure fixed positioning of the control module with respect to the nozzle body it is advantageous if the piston guide comprises at least three projections directed radially outwards which lie adjacent to side surfaces of the nozzle body that are formed by the drilled hole. The spaces between the projections form channels for the fuel.
The projections can run along the entire axial length of the piston guide.
It is however advantageous if the piston guide in the area of the upper end of the drilled hole is spaced from the side surfaces of the needle body, which are formed by the drilled hole, such that an annular channel is formed for the fuel. In this case, the projections are merely arranged in a lower section of the piston guide. In this case, the bypass restrictor can lie adjacent to a bypass drilled hole arranged in the control module, which bypass drilled hole opens out into the annular channel. Rapid transportation of the fuel from the high pressure inlet into the bypass drilled hole is guaranteed as a result of the annular channel. The advantageous aspect of such an arrangement is the fact that the bypass drilled hole is spaced from the high pressure inlet and that consequently the construction space in the control module is better utilized. Walls around the high pressure inlet or around the bypass drilled hole that are too thin for a high pressure are also avoided as a result.
The projections on the piston guide are preferably arranged symmetrically around the axis of the drilled hole.
In order to reduce the resistance to flow of the fuel, it lies within the scope of the invention to provide a radial projection for the drilled hole which extends at least over the axial length of the piston guide and into which the high pressure inlet opens out. In this case, projections for the piston guide are not required but are possible.
The inlet restrictor can be connected directly to the high pressure inlet.
Alternatively, the inlet restrictor is connected to the annular channel, in other words it is connected only indirectly to the high pressure inlet whereas the bypass drilled hole is connected directly to the high pressure inlet.
In order to simplify the manufacturing process, it is advantageous for the nozzle needle and the control piston to be formed as a single piece. In this case, the piston guide is used as a guide both for the control piston and also for the nozzle needle.
In order to reduce the resistance to flow of the fuel in the drilled hole, it is advantageous for the nozzle needle to be spaced from side surfaces of the nozzle body which are formed by the drilled hole.
The dimensions of the control piston are adapted to the dimensions of the piston guide in such a way that no channel is produced for the fuel inside the piston guide.
Alternatively, the nozzle needle and the control piston are not formed as a single piece. In this case a needle guide, adjacent to which is located the nozzle needle such that at least one channel is formed for the fuel flow, is provided inside the drilled hole.
In order to increase the closing force of the nozzle needle, it is advantageous for a spring to be provided in the drilled hole, which pre-tensions the nozzle needle in a downward direction. For example, the nozzle needle comprises a spring plate, whereby the spring is tensioned between the spring plate and the lower end of the piston guide.